Assassin is the
Assassin-class Servant of Caster
in the Fifth Holy Grail War of Fate/stay night. He is an irregular
Servant that shouldn't exist according to the rules of the Holy Grail War, but
Caster's unique nature as a Servant summoning a Servant twists the process.
The true Servant, one of the Hassan-i-Sabbah, is summoned as True Assassin in
the Heaven's Feel route.